1. Field of the Invention
The present invention relates to methods that allow to the formation of channels in sonar devices. It applies more particularly to towed-array sonar devices, in which the data must be transmitted by the towing cable whose transmission rate is necessarily limited.
2. Discussion of the Background
As shown in FIG. 1, a simple sonar array comprises a number of hydrophones 1 through N distributed with a pitch d along a line segment.
The signals received by these hydrophones (more generally by these sensors) are combined both to obtain a received signal stronger than that from an isolated hydrophone and to derive data about the direction of the source transmitting the signal being received.
If the signals from all the hydrophones are simply summed, a directivity pattern is obtained which exhibits a direction of best reception perpendicular to the line segment along which the hydrophones are aligned. This best direction is referred to as a receive channel.
In order to be able to monitor a sector wider than that defined by this single front channel, it is known to sum the signals from the hydrophones with a phase shift which allows one to obtain channels in directions which depends on this phase shift.
As a matter of fact, the signals received from a source located in a direction .theta. with respect to the normal to the array exhibit a phase difference, at the frequency f, which is given for a sensor i with respect to the sensor 1 located on the far left in the Figure, by the formula: ##EQU1##
Thus if phase shifts are applied to the signals from the sensors, at an operating frequency f.sub.0 given for a sensor i by the formula: ##EQU2## a directivity pattern is obtained which exhibits a maximum for the direction .theta..sub.0 with respect to the normal to the antenna. A channel is thus formed in this direction .theta..sub.0 and it is possible to form as many channels as desired, within the limits defined below, by applying adequate phase shifts.
The received signals are rarely pure frequencies, and when f is different from f.sub.0, a rotation of the direction of the channel is observed which is given by the formula: ##EQU3##
With regard to the orientation of the channel allowing to determine the direction of the source of the signal, there is thus an error in this direction being obtained. This error is more or less tolerable, and a common rule consists in admitting that it can attain a value equal to the half-width of the lobe at -3 dB of attenuation of the channel signal. This width .theta..sub.-3 is given, with c being the velocity of propagation of sound waves in water and L the length of the array (equal to the distance between the end hydrophones), by the formula: ##EQU4##
With the bandwidth of the received signal being equal to B, the maximum frequency shift is equal to B/2.
For a frequency change .delta.f, the degree of rotation of the lobe is obtained by deriving the formula (3) for f=f.sub.0 : ##EQU5##
Putting .delta.f=B/2 and equating (sin .theta.) to sin .theta..sub.-3, we obtain the following basic condition: ##EQU6##
By noticing that the first term of this formula is equal to the distance P in FIG. 1, known as the depth of the array, this formula can be written as: ##EQU7##
The distance, or range, resolution is given by the expression c/2B which determines the band of frequencies to be used to achieved the desired resolution.
It can thus be seen that the monitored sector determined by the basic condition obtained above, i.e., .theta..sub.0 maximum, is more narrow the higher the range resolution.
It is known, to increase this sector of observation, to use a so-called "delay+phase shift" technique in which the sector of observation is divided into n subsectors in which the basic condition can be fullfilled. To obtain these n subsectors, the array is oriented in an electronic manner toward the center of each of these subsectors by applying predetermined delays to the signals from the hydrophones. Within each of these subsectors, the phase-shift technique is used as seen above to obtain adequate channels.
When this operation is performed in the digital manner, which tends to become the general case, the signals from each of the hydrophones are sampled and then digitized, which leads to transmitting to the processing circuitry a considerable number of digital data.
In the case of a towed-array sonar device, the weight and the volume of the towed portions are reduced as much as possible by limiting them to the array and to a minimum number of processing circuits, that is most often to the sampling circuits and the analog-to-digital converters. The signals thus obtained are transmitted, most often in serial form, by connections included in the towing cable of the towed vehicle which contains the array and said circuits. It is then very difficult to transmit all these signals through these connections, in particular through connections of the coaxial type whose transmission rate is limited to a few dozens of megabits per second.